Filter for determination of mercury in exhaust gases

ABSTRACT

An apparatus, process, coating, and filter for the accurate measurement of total mercury concentration in flue gas. In a preferred aspect, the concentrations of both elemental and oxidized mercury are preserved by the apparatus for analysis. Accordingly, embodiments of the present apparatus and process can be used to determine regulatory compliance or for process control measurement.

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/512,093 filed Oct. 20, 2003, the contents ofwhich are hereby incorporated by reference in their entirety.

FIELD OF THE INVENTION

The present inventive subject matter relates generally to an apparatus,process, coating, and filter for accurately measuring total gaseousmercury concentration in flue gas. In one aspect, the preferredembodiments preserve the true concentrations of both elemental andoxidized mercury in flue gas for analysis. Accordingly, the preferredembodiments can be used to determine regulatory compliance or forprocess control measurement.

BACKGROUND OF THE INVENTION

Mercury (Hg) has been designated a toxic compound found in theenvironment by the EPA. The Draft EPA Mercury Study Report to Congressestimates total annual mercury emissions from anthropogenic sourcesglobally is 4,000 tons, with 200-300 metric tons emitted annually in theU.S. The report identifies the largest sources of mercury emissions inthe U.S. to be utility boilers, followed by waste incinerators whichcombust mercury-containing wastes (municipal and medical), coal-firedindustrial boilers, and cement kilns that combust coal-based fuels.Other potentially important sources of mercury emissions aremanufacturing plants and basic chemical processes. One particularlynotable source of mercury emissions is coal-fired power plants. Theseplants emit 48 tons of mercury per year, and will be required to reducethis emission level by greater than 90% by 2010.

The EPA published regulatory guidelines for mercury emissions frommunicipal waste combustors in 1995. To quantify the emissions from eachpoint source, a mercury CEMS (Continuous Emissions Monitoring System)will be required. There are three forms of mercury in smoke stackeffluent gas, or flue gas, from a coal fired power plant that canpotentially be monitored by a mercury CEMS, namely elemental Hg⁰,oxidized Hg⁺², and particulate bound Hg of either species, at stack gastemperatures in excess of 200° F. However, the EPA does not currentlyrequire the continuous monitoring of particulate bound Hg⁰. Accordingly,total mercury continuously monitored in accordance with EPA regulations,i.e., gaseous mercury, is the sum of elemental mercury (Hg⁰) andoxidized mercury (Hg⁺⁺). However, Massachusetts and Wisconsin plan toregulate particulate bound Hg⁰ starting in the year 2006.

Mercury in both of these gaseous forms is very sticky chemically, havinga strong affinity to adsorb onto a wide variety of surfaces, andextremely difficult to handle and transport through an extractive gassampling system to a gas analyzer for measurement. Since flue gasesusually contain very low levels of gaseous mercury that must bedetected, the small amount present that readily adsorbs onto surfaces oftubing, valves, and other fittings distorts any reading made.

Further, the more restrictive controls on air toxic mercury mandated bythe EPA will likely result in higher operational costs to flue gasgenerators, such as coal-fired plant owners. Accordingly, there exists areal and eminent need for the development of a durable, low cost,accurate technology capable of measuring mercury species and totalmercury emitted in a smoke stack effluent gas in real-time. A totalmercury measurement is required for regulatory monitoring, whereas theevaluation of mercury control technologies and manufacturing processesrequires measurements that reveal the distribution of elemental andoxidized mercury.

Various approaches in the development of CEMS's for mercury measurementin flue gas emissions to determine the amount of elemental or reducedmercury (Hg⁰) therein have included x-ray diffraction, UV photometry,cold vapor Atomic Absorption, and atomic fluorescence methods. Somemethods detect and measure amounts of elemental or reduced mercury atexcitation wave lengths of 254 nm. Unfortunately, any oxidized speciesof mercury (Hg⁺⁺) therein cannot likewise be measured.

In order to measure total mercury, the oxidized mercury species state(Hg⁺⁺) must first be reduced to Hg⁰. The predominant form of oxidizedmercury existing in flue gas coal fired combustors, waste combustors,and incinerators is HgCl₂ (C. S. Krivanek, III, Journal of HazardousMaterials, 47 (1996) Mercury Control Technologies for MWC's: TheUnanswered Questions, pp. 119-136 and IEA Coal Research, MercuryEmissions and Effects—The Role of Coal). It is presently known that bymeasuring the elemental mercury concentration in a gaseous matrix bothprior and subsequent to a mercury reduction process, the total mercuryconcentration and the distribution of oxidized and reduced mercury canbe determined. The determination of this distribution is of significantvalue, and often essential, in the development of effective mercurycontrol options for gaseous emissions and can have value to effectiveprocess quality control.

Typically, in order to measure the total mercury concentration of asample by laboratory analysis, the reduction of oxidized mercuryinvolves the mixing of a gas or liquid sample with reducing solutionsprior to measurement of Hg⁰. Similarly, several instrument manufacturershave incorporated reducing solutions into their on-line CEMS's formercury measurement in the exhaust gasses of flue stacks. These devicesrely on reducing solutions such as sodium hydroboride solution, stannouschloride solution, or other reducing solutions to convert oxidizedmercury to Hg⁰ prior to measurement by a detector such as an ultraviolet(UV) atomic absorption (AA) or atomic fluorescence detector. An obviousdisadvantage to this type of instrument design is that it requiresfrequent solution replenishment.

Further, a dry mercury reduction method is preferable to a wet one incontinuous on-line measurement since there are fewer maintenancerequirements, which, in turn, translates to a more reliable technique.Thermal reduction of oxidized mercury is such an alternative dry method;it is reported in the open literature that oxidized mercury can easilybe reduced at temperatures of about 800° C. However, a gaseous exhaustfrom a coal-fired boiler or incinerator may often contain oxidizingagents that can and will reoxidize the thermally reduced mercury beforea mercury measurement can be effected.

For example, the fossil-fueled and waste combustion industries generategaseous mixtures which typically contain compounds such as NO_(x), O₂,H₂O, CO₂, and CO. Other gases such as SO₂, HCl, Cl₂, H₂S, NH₃, andvolatile metals and organics also may be present depending on the typeof fuel combusted. Of these components, hydrochloride gas and oxygentypically have a noticeable effect on the reoxidation of elementalmercury. Furthermore, the presence of oxygen in admixture with HCl gasacts to enhance the hydrochloride effect on elemental mercury oxidation.

The effect of hydrochloride on the reoxidation of elemental mercury uponthermal reduction is also reported by Wang et al., “Water, Air and SoilPollution” 80: 1217-1226, 1995. Wang et al. used crushed quartz chips tofill a quartz cell and thereafter heated their cell to 850°-900° C. toreduce Hg⁺⁺ in a gaseous stream. They also found the addition of HCl tothe gaseous matrix negated the converter effect. Their approach tocounter the HCl effect was to fill the converter with basic materials.Filling the quartz converter cell with a layer of soda lime, sodiumcarbonate, or crushed quartz treated with NaOH solution improved theoverall conversion efficiency by reacting the basic materials fillingthe conversion tube with hydrochloride (HCl) gas and preventing thereoxidation of elemental mercury.

The effectiveness of these approaches was limited, however, due to thesevere corrosive nature of the basic solids. Further, the hightemperatures necessary for the conversion that Wang et al. reporteddestroyed their converter cells within two days. To solve theseproblems, the use of an inertial filter to separate mercury vaporspecies from the particulate in a stack gas stream has been proposed.

There are numerous prior art inertial filter systems known for use inprocessing extractive gas sample conditioning systems. The inertialfilter itself is an invention of the Bendix Corporation (U.S. Pat. No.4,161,883) based on work done by Carl Laird. Mott MetallurgicalCorporation has since offered an entire line of inertial filters forvarious applications. Accordingly, these inertial bypass filters havebeen used in the art for many years. The construction of these priorinertial filters is a porous tube within a solid tube with a very smallannular space in between the two tubes.

The basic principle of operation of the inertial filter is to acceleratethe particulate material contained in the process gas in a vectordirection with sufficient velocity to prevent the particles fromsticking to the walls of the sampling tube. This enables the extraction,at a 90° angle, of a small aliquot sample at very low face velocity, fortransportation to a gas analyzer. The basic principle is to provide a70-100 fps (feet per second) gas velocity down the center of the poroustube at a flowrate sufficient to prevent the majority of the particulatematter from adhering to the porous tube and without penetration throughthe porous tube. The flow rate is dependent upon the gas density,temperature, diameter of the sampling tubing, absolute pressure, andparticulate loading.

Particles subjected to a velocity of 70-100 fps continue to travel inthe straight vector direction, and the sample aliquot is withdrawnaxially, at a very low filter face velocity of 0.005 fps, separating thesample aliquot from the initial particulate material. The center boretubing is typically made from sintered stainless steel, available invarious micron sizes, made to order. The micron size chosen for thisapplication is usually about 0.5 microns.

It is generally known that these prior art inertial bypass filters areheated to prevent condensation of the analyte. It is further known thateither direct or indirect heating can be used in this regard. Currentstate of the art sample acquisition systems, then, rely on heatedfilters to extract flue gas from the flowing process, remove particulatematerial, and transport the clean sample to the sample conditioningsystem for analysis.

U.S. Pat. No. 6,475,802 attempted to combine this concept of using aninertial filter with the use of quartz chips taught by Wang et al. toprepare an improved process for measuring and detecting total gaseousmercury concentration in flue gases. This patent teaches a module fordetection of amounts of gaseous mercury in both ambient air and fluegases. The module uses packed quartz chips to adsorb elemental andoxidized gaseous mercury to prevent their removal from a flue gas streamupon passage through a filter to remove unmeasurable particulates.Accordingly, this patent requires a denuder to strip the gaseous mercurycomponents from a gas sample before the sample is passed through afilter to remove the particulates.

The inertial bypass filter apparatuses presently known in the art forcontinuously measuring quantities of mercury in smoke stack effluent gasfurther have the disadvantage of having to maintain a fairly constanttemperature at about 200° C. to provide anything resembling an accuratemeasurement. However, these prior art apparatuses are unable to make avery precise measurement of the amount of mercury in the flue gas.

Additionally, these prior art apparatuses alter the flue gas in order totake an adequate measurement of the amount of mercury contained therein.Accordingly, these gases do not output the prior art apparatuses in thesame state in which they entered the apparatuses. However, the mercuryremoval processes generally employed by the facility operator willrequire that the concentrations of elemental and oxidized mercuryspecies be preserved by the sampling system for analysis, i.e., the fluegas cannot be altered. In other words, to achieve good analyticalresults, the filtering process at the sample point should not change thespecies of mercury existing in the sample and should not attenuate(adsorb) the mercury.

Another disadvantage is that these prior art filters coat up withparticulate material, attenuating the mercury concentrations transportedacross the filter. Blowback is employed to periodically remove theparticulate coating on the filter, by back purging dry, compressed airunder high pressure (100 psig). However, Hg⁰ is oxidized to Hg⁺² acrossthe filter media, caused in part by reaction with artifacts in theparticulate matter coating on the filter media as well as reaction withthe filter media itself.

Accordingly, typically it has been difficult to use known apparatuses tosample total mercury concentration in flue gas without altering theratios of oxidized to elemental mercury and without attenuating theconcentrations of either species. A standard inertial filter typicallywill not yield good results when sampling mercury species in flue gas.Due to its chemical nature, Hg⁰ and Hg⁺² are difficult to transportacross a filter media. Typically, the filter media must be held at atemperature of 400° F. or better, and the filter media isolated from themercury species to prevent oxidation of elemental Hg and to avoidreduction of oxidized Hg to elemental.

BRIEF SUMMARY OF ASPECTS OF THE INVENTION

The present inventive subject matter relates generally to a process,apparatus, coating, and filter for the measurement of total gaseousmercury concentration in flue gas. In one preferred aspect, the presentprocesses and apparatuses are capable of improving the accuracy of thetotal gaseous mercury measurement required by the EPA by minimizing theloss of oxidized mercury through adsorption (and absorption) of thegaseous mercury in the gas sample on the particulate removal filter andother metal surfaces in the apparatus used for sampling mercury. Inaddition, preferred processes and apparatuses also enable themeasurement of the individual concentrations of elemental mercury andoxidized mercury by preventing the catalyzed oxidation of elementalmercury on metal surfaces of the apparatus including the filter, andpreferably also other metal surfaces, at the desired operatingtemperature of the sampling system.

Preferably, the present subject matter addresses one of the manysample-conditioning components necessary to transport gaseous mercury inboth forms (Hg⁰ and Hg⁺²) to the gas analyzer. Preferred aspects relateto sample acquisition; the first components to contact the extractedprocess gas; the stinger (sampling probe inserted into the process gas);and the sample gas filtering system used to separate the particulatematerial found in the process gas from the Hg⁰ and Hg⁺² vapor species tobe analyzed.

These preferred aspects are achieved herein by rendering passivemetallic surfaces of a sintered metal filter media used in an inertialfilter to prevent any catalysis or deposition of gaseous mercury thereonduring analyte transport through the filter. This is preferably achievedby spherically coating each individual metallic particle in the sinteredmetal filter media.

A preferred embodiment relates to a process for accurately measuringtotal gaseous mercury concentration in flue gas comprising the steps of:

-   -   (1) removing a gas sample from flue gas containing gaseous        mercury using a gas sample acquisition probe;    -   (2) passing the gas sample through a porous filter element in an        inertial filter to remove particulate material present in the        gas sample; and    -   (3) measuring the amount of gaseous mercury present in the gas        sample.

In this embodiment, all metal surfaces of the inertial filter are coatedwith a protective coating to prevent chemical reactions between themetal surfaces and the gas sample.

Another preferred embodiment relates to an apparatus for collecting agas sample of flue gas containing gaseous mercury and accuratelymeasuring the total gaseous mercury concentration therein comprising:

-   -   (1) a gas sample acquisition probe mounted into the flue gas;    -   (2) a porous filter element inside an inertial filter that        removes particulate material from the gas sample and outputs a        particulate free gas sample;    -   (3) an analyzer for measuring the amount of gaseous mercury        present in the particulate free gas sample; and    -   (4) sample transport tubing used to transport the gas sample        from the probe to the inertial filter, and the particulate free        gas sample from the inertial filter to the analyzer.

In this embodiment, each of the probe, inertial filter, and sampletransport tubing contain metal surfaces that come into contact with thegas sample or the flue gas. The metal surfaces of this inertial filterare in turn coated with a protective coating that reduces chemicalreactivity to the gaseous mercury, but does not impede gas flow throughthe inertial filter.

Yet another preferred embodiment relates to a protective coating onmetal surfaces of a gas sample acquisition and filtration apparatus forcollecting a gas sample of flue gas containing gaseous mercury andremoving particulate material therefrom. This protective coating reduceschemical reactivity on the metal surfaces to the gaseous mercury, butdoes not impede gas flow through the apparatus. In this regard, thecoating is on all metal surfaces of the apparatus coming into contactwith the gas sample or the flue gas.

Still another preferred embodiment relates to an inertial filter forremoving particulate material from a gas sample of flue gas containinggaseous mercury without removing or chemically altering the gaseousmercury. The inertial filter comprises a stainless steel housing havinga metal surface and a porous filter element having a metal surface toremove the particulate material from the gas sample. All metal surfacesof the inertial filter coming into contact with the gas sample arecoated with a protective coating that reduces their chemical reactivityto the gaseous mercury, but does not impede gas flow through theinertial filter.

It is to be understood that a given embodiment need not meet all aspectsor provide all the disparate features noted above. On the contrary, thescope of the invention is measured by the claims as issued and not bythis brief summary of aspects of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments are disclosed in connection with the attachedFigures in which:

FIG. 1 is a filter flow diagram of the present inertial filterapparatus. Included is an exploded view showing that the surfaces comingin contact with the gas sample are coated with fused silica.

FIG. 2 is a diagram of the present inertial filter apparatus, includinga cross sectional drawing thereof.

FIG. 3 is a diagram of the present static filter housing including amounting flange and stringer that extends into the stack.

FIG. 4 is a door-side view and bottom view of the present gas sampleacquisition and filtration apparatus.

FIG. 5 is a door-side view of the present gas sample acquisition andfiltration apparatus showing addition of a diluter assembly and anaccumulator tank.

FIG. 6 is a door-side view and bottom view of an enclosure and mount forthe present gas sample acquisition and filtration apparatus.

FIG. 7 is a plumbing diagram showing a sectional view of the staticprobe and three different plumbing arrangements for the static probe.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Definitions

As used herein, the terms “elemental mercury”, “elemental Hg”, “metallicmercury,” “reduced mercury”, “non-ionized mercury vapor”, “Hg(0)”, and“Hg⁰” all mean and refer to the same form of mercury.

As used herein, the terms “oxidized mercury”, “oxidized Hg”, “reactivegaseous mercury”, “ionic mercury”, “Hg(II)”, “Hg⁺⁺”, and “Hg⁺²” all meanand refer to the same form of mercury.

As used herein, “gas sample” means a portion of gas removed from fluegas flowing anywhere in a combustor's process by the preferredapparatus. Once removed from the flue gas, particulates are removed fromthe gas sample, which is then analyzed and measured to determine totalgaseous mercury concentration therein, corresponding to the totalgaseous mercury concentration of the flue gas.

A preferred apparatus may contain the parts of a Sample Acquisition orGas Sampler, Sample Transport, Sample Conditioning, and Flow Control/GasManifold to provide clean, particulate free, wet or dry flue gas samplesfor analysis by a gas analyzer and data recorder.

As used herein, “sample acquisition” refers to the active removal of agas sample from flue gas flowing anywhere in a combustor's process. The“sample acquisition”, “gas sample acquisition probe”, or “gas sampler”portion of the apparatus is typically a removable probe (stinger)mounted into the flowing gas process.

As used herein, “sample transport” refers to those portions of thepreferred apparatus that transport, or convey, a gas sample throughoutthe apparatus. Typically, sample transport is conducted through the useof tubing.

As used herein, “analyzer” or “gas analyzer” refers to a piece ofequipment known in the art as capable of measuring total gaseous mercuryconcentration in a gas sample.

As used herein, “sample conditioning” refers to the removal ofparticulates from the gas sample, permitting measurement of theconcentration of total gaseous mercury in the gas sample by the gasanalyzer. Preferably, the “sample conditioning” portion of the apparatuscomprises an internal or external filter assembly for immediate removalof particulate material. Accordingly, a preferred apparatus uses aninertial filtering system for sample conditioning.

Inertial Filter

The preferred apparatus contains an inertial filter to removeparticulates from the flue gas sample before measurement for totalgaseous mercury concentration. The inertial filter is capable ofremoving the particulate material in the flue gas sample, yet permitsthe passage of both forms of gaseous mercury in chemically unalteredform for subsequent analysis. The inertial filter, and the apparatus asa whole, does not oxidize elemental mercury or reduce oxidized mercury,so that a user may determine how much mercury in the flue gas is in eachstate.

Generally, inertial filters are constructed as a porous tube within asolid tube with a very small annular space in between the two tubes. Thebasic principle of operation of the inertial filter is to accelerate theparticulate material contained in the gas sample in a vector directionwith sufficient velocity to prevent the particles from sticking to thewalls of the sampling tube. This enables the extraction, at a 90° angle,of a particulate-free gas sample, at very low face velocity, fortransportation to a gas analyzer. The basic principle is to provide a70-100 fps (feet per second) gas velocity down the center of the poroustube at a flowrate sufficient to prevent the majority of the particulatematter from adhering to the porous tube and without penetration throughthe porous tube. The flow rate is dependent upon the gas density,temperature, diameter of the sampling tubing, absolute pressure, andparticulate loading. The ratio of straight vector particle velocity tothe radial (axial) gas sample velocity should be greater than 10,000:1.In a preferred embodiment, the ratio is about 14,000:1 (i.e., about 70fps/0.005 fps).

Particles subjected to a velocity of 70-100 fps continue to travel inthe straight vector direction, and the gas sample is withdrawn at a verylow filter face velocity of 0.005 fps, separating the gas sample fromthe initial particulate material. The center bore tubing, or porousfilter element, is preferably made from a sintered metal filter media,available in various micron sizes, made to order to remove particulatematerial from the gas sample. The micron size chosen for thisapplication is usually from about 50 to about 0.1 microns. In apreferred embodiment the final micron size of the inertial filter usedin the present apparatus is about 0.5 microns. In a particularlypreferred embodiment, the sintered metal filter media is sinteredstainless steel. Further, the porous filter element may have an innerdiameter of between approximately ¼″ and ½″, preferably approximately ¼,⅜″, or ½″, and a length of approximately 12 to approximately 24 inches.

Stainless steel or other metals are desirable as the material ofconstruction for the housing, or solid tube, of the inertial filter andassociated structure that transports the gas sample to the analysissystem due to its mechanical ruggedness and ability to withstand thehigh temperatures that are necessary to prevent condensation. Stainlesssteel, however, typically catalyzes the oxidation of elemental mercuryto oxidized mercury. It also has a propensity to adsorb and desorboxidized mercury. Other materials such as glass, quartz, or Teflon® willnot do either of these and would be much better than untreated stainlesssteel, although with these materials problems can arise such asbrittleness in the case of glass and quartz and mechanical instabilityin the case of Teflon® and other plastics.

In an alternative preferred embodiment, the inertial filter can be madeup of a ceramic rather than a metal such as stainless steel. While notas durable as stainless steel, a ceramic filter avoids the problems ofcatalyzing the oxidation of elemental mercury to oxidized mercury andadsorption and desorption of oxidized mercury. Accordingly, a ceramicinertial filter does not require a treatment or coating as does astainless steel inertial filter described herein to still be effectivein transporting gaseous mercury.

Protective Coating

The preferred inertial filters solve these difficulties by incorporatinga protective coating on all metal surfaces coming in contact with thegas sample. The protective coating can be coated on the metal surfacesby processes well known to those of ordinary skill in the art. Inpreferred embodiments, the metal surfaces coated with the protectivecoating are comprised of stainless steel. This coating stops thecatalysis of mercury oxidation and drastically reduces the adsorption ofoxidized mercury.

In a preferred embodiment, the non-metallic coating can be comprised ofany material capable of coating the individual metallic particles of theinertial filter, which is stable at an operating temperature of 450° F.or less, and which produces a finished filter porosity of 0.1 microns orgreater. Specific examples of materials useful in forming the protectivecoating on the metal surfaces coming in contact with the gas sample arethose selected from the group consisting of fused silica (a form ofquartz), a fused silica derivative, a fused silica-like material,silica-oxide, Teflon®, Viton, Silicone, Silcosteel®, Deactivated FusedSilica Lined®, Silane Deactivated Fused Silica, Silane, Sulfinert™,Siltek®, Passivation, Passivation Coatings, Quartz, Glass, Ceramic,Plastic, Restek®, Titanium, Titanium Derivative, Polymer, Copolymer,Magnesium, Magnaplate, Nedox®, Urethane, Buna, Kalrez®, Chemraz®, Aegis,Neoprene, Flouro Silicone, Elastomer, Latex, Rubber, Isoprene,Butadiene, Styrene, Butyl, Ethylene, Propylene, Nitrile,Epichlorohydrin, Hypalon, Polysulfide, Silicone Polymer, FluorocarbonPolymer, Acrylic Ester, Acrylic Halide, and combinations thereof. In aparticularly preferred embodiment, the protective coating on the metalsurfaces is comprised of a thin layer of fused silica or a derivativethereof.

In particular, silica and its derivatives are among the few materialscapable of putting a coating with a thickness of up to approximatelyseveral thousand Angstroms (a few molecules thick) on the metal surfacesof the inertial filter in order to prevent gas sample contact with thesurface but not impede gas sample flow through the filter. Accordingly,in a preferred embodiment the coated inertial filter will have a filterporosity of about 0.5 microns. The present preferred apparatus, then, iscapable of measuring the individual concentrations of elemental mercuryand oxidized mercury in a flue gas sample by preventing the catalyzedoxidation of elemental mercury on the metal surfaces of the apparatus,such as the porous filter element, at the desired operating temperatureof the sampling system.

The protective coating isolates the gaseous mercury species, both Hg⁰and Hg⁺², from the porous filter element and transports the Hg⁰ and Hg⁺²vapor without any loss across the filter, retaining the Hg⁰ to Hg⁺²concentration ratio existing in the flue gas. Accordingly, theprotective coating prevents chemical or physical alteration of the gassample passing through the inertial filter. This permits the accuratemeasurement of the Hg⁺² and/or the Hg⁰ gaseous mercury species.

The protective coating serves to reduce chemical reactivity of the metalsurfaces coming in contact with the gaseous mercury to said gaseousmercury, without impeding gas flow through the inertial filter. Suchchemical reactions reduced on the metal surfaces include, withoutlimitation, oxidation of elemental mercury to a chemically combined formof mercury and loss of oxidized mercury through adsorption on the metalsurfaces.

As it passes over quartz or other non-catalytic surfaces, the oxidationof elemental mercury to oxidized mercury, aided by the presence ofchloride radical (Cl⁻), takes place at a much higher temperature than itdoes when passed over a metal surface, such as those materials that makeup stainless steel. By coating the surface of the metal parts of thesampling system with for example, fused silica or its derivatives, thepreferred apparatus prevents the mercury from contacting the metalsurface where its oxidation will be “catalyzed” by the metal. Thus, thestate of the mercury in the sample is preserved for subsequent analysis.

Further, elemental mercury has a tendency to be adsorbed by certainmetal surfaces, i.e., attach itself to these metal surfaces such asstainless steel. Accordingly, by coating the metal surfaces of thepreferred apparatus with a film of, for example, fused silica orderivatives thereof, the elemental mercury will bead up and quicklyleave the surface since it cannot adsorb to the metal surface. Mercuryis repelled by the fused silica or derivative thereof, rather thanattracted to the metal surface, and can pass by the metal, remaining inthe gas phase for subsequent analysis.

These two phenomena are related in that, in most catalytic processes,the intimate contact of the material with the catalyst is necessary andadsorption is often involved in catalytic processes. The fused silica orfused silica derivative coating is a barrier that prevents either formof mercury from getting in contact with the metal surface.

The porous filter element of the preferred apparatus, if comprised of ametal rather than a ceramic, must bear this protective coating, such asa coating of fused silica or a derivative thereof, because it has thehighest surface area. However, particularly preferred embodimentsfurther contemplate that all metal surfaces of the apparatus that comeinto contact with, or are wetted by, the flue gas sample, or the fluegas itself, are coated with a protective coating that reduces chemicalreactivity of all of these metal surfaces to gaseous mercury.Accordingly, components of the apparatus that may bear such a coatingmay include but are not limited to the following:

-   -   1. stinger (sampling probe inserted into the stack, duct, or        pipe);    -   2. sintered metal filter media used for primary particulate        separation;    -   3. sample transport stainless steel tubing;    -   4. tube fittings used for sample tubing interconnections;    -   5. gas sampling pump for transporting the sample under pressure;    -   6. flow control valves for controlling flow;    -   7. heated stainless steel transport bundle for transporting the        sample gas to the sample conditioning system;    -   8. stainless steel impingers within a thermo electric or        refrigeration sample cooler; and    -   9. solenoid gas control valves.

Accordingly, the protective coating enables the preferred apparatusesand processes to provide near real time, on-line monitoring of metalemissions and chemical emissions in flue gases and for archiving of thesample results. The preferred apparatus and process, then, can be usedto monitor flue gases from a number of entities, including, but notlimited to, furnaces, incinerators, smelters, iron and steel plants,lime and cement kilns, battery plants, and semiconductor plants. Anexemplary application is the analysis of fly-ash from coal combustion.

Processes of Preparing Protective Coating

A preferred process for coating the apparatus with, for example, fusedsilica or a derivative thereof bonds a layer of fused silica or a fusedsilica derivative to the sintered metal filter media, and all othermetal surfaces coming into contact with the gas sample during sampletransport. This process bonds thin, uniform, flexible layers (up toseveral thousand angstroms) of fused silica or its derivatives to thesurface of the metal surfaces in the preferred apparatus. The fusedsilica coating is as inert as Teflon® or a glass composite filter media,and it can be used to transport gaseous mercury without chemicallyaltering it at operating temperatures exceeding 400° F.

For example, a preferred process for coating the sintered metal filtermedia would start out with a sintered filter of large pore size, such asabout 50 microns. Layers of the fused silica coating are thenprogressively applied to the sintered filter. After the coating iscompleted, the coated sintered metal filter media should preferably havean about 0.5 micron pore size. This coating process is preferablyconducted using a gas phase coating process in order to permit thespherical coating of each individual filter particle, rendering eachparticle benign.

Passivation of the fused silica layer can be improved further by addinga secondary chemical deactivation layer. The purpose of the chemicaldeactivation layer is to cover any silanol (—Si—OH) groups on thesurface of the coating of fused silica or a derivative thereof. Thestandard deactivation chemical used by industry is an intermediatepolarity siloxane containing phenyl and methyl moieties. Silanol isnormally not a problem when transporting mercury through the porousfilter element, but under certain applications, a secondary coating onthe fused silica is necessary.

In another preferred embodiment, all metal surfaces of the apparatuscoming in contact with the flue gas are similarly coated with a polymercoating. In this embodiment, thin, uniform, flexible layers (up toseveral thousand angstroms) of the polymer are bonded to the sinteredmetal filter media, and all other metal surfaces coming into contactwith the gas sample during sample transport. The polymer coating can beused to transport gaseous mercury without chemically altering it atoperating temperatures below 400° F. The polymer coating is preferablyapplied using a gas phase coating process in order to permit thespherical coating of each individual filter particle, rendering eachparticle benign.

Preferred Apparatus Components

A filter flow diagram of a particulate laden stream 18 through thepreferred inertial filter 10 is shown in FIG. 1. The inertial filter 10comprises an outer filter housing 11 made up of a housing annulus 12 anda porous wall 13. Inside the filter housing 11 is a porous filterelement 14 having a fused silica coating 15 of molecular thicknesssurrounding the filter particles 16, leaving interstitial spaces 17 ofapproximately 0.5 microns each between each coated filter particle. Aparticulate laden stream 18 enters the inertial filter 10 and travels inthe filter air flow direction 19. Also present are a blowback air inlet20, a calibration gas input 21, a sample output 22, and a filter surfacetemperature thermocouple 23.

Another schematic depiction, as well as a cross-sectional view, of thepreferred inertial filter is shown in FIG. 2 as numeral 10. The outerfilter housing 11 of the inertial filter 10 is made from a stainlesssteel pipe 40 having an approximately 0.625″ outer diameter and anapproximately 0.527″ inner diameter. Four 0.25″ holes 41 are located onthe underside of the stainless steel pipe 40. These holes 41 are cappedwith ¼ compression pieces 42. Unions 43 are included on either end ofthe stainless steel pipe 40. The cross-sectional view shows the outerfilter housing 11, the housing annulus 12, the porous wall 13, and theporous filter element 14. All parts of the inertial filter 10 coming incontact with a gas sample are coated with a fused silica coating 15.

A door-side view and bottom view of the preferred apparatus is shown inFIG. 4. A gas sample acquisition probe 100 is shown to contain astainless steel stinger tip 150 joined to an aluminum stock 151 by astainless steel compression unit 152. The gas sample acquisition probeis attached to a stainless steel mounting flange 156, and thus to theapparatus enclosure 155, via two couplings 153 and 154. The twocouplings 153 and 154 are attached to the flange 156 via holes 169 and170. The gas sample is transported into the inertial filter 10 by aninlet stainless steel sample tube 157, attached via a reducing union158. A spike calibration gas tap 171 is located on the inlet sample tube157 before attachment to the inertial filter 10. The gas sample leavesthe inertial filter 10 via a stainless steel pipe 159, attached via areducing union 160. The gas sample then passes through a venturi flowmeter 161 under vacuum pressure. Finally, the gas sample leaves theapparatus enclosure 155 by passing through an eductor 162 and a maleconnector 163. The gas sample is powered through this system via asample pump 164 and a needle valve 165. The temperature of the gassample is maintained via a radiant heater 166 and/or a pre-heater 167.Attached to the apparatus enclosure 155 is a main control enclosure 168.Preferably, all parts coming in contact with a gas sample are coatedwith a fused silica coating 15.

A door-side view of another preferred apparatus is shown in FIG. 5. Inaddition to the items shown in FIG. 4, in this embodiment the inertialfilter 10 has a sample output 200 entering a secondary dilutor assembly201. An accumulator tank 202 is fitted to provide high volume blowbackair for fittings 20 and 171. All parts coming in contact with a gassample are coated with a fused silica coating 15.

A door-side view and bottom view of the apparatus enclosure 155 andmounting flange 156 for the preferred apparatus is shown in FIG. 6. Themounting flange 156 is connected to the apparatus enclosure 155 via apipe 225.

Static Filter

An apparatus using a static filter rather than an inertial filter toremove the particulates from the flue gas sample before measurement ofthe total gaseous mercury concentration is further contemplated herein.As with the inertial filter, the static filter will similarly contain aprotective coating on all metal surfaces that may come into contact withthe gas sample. In a particularly preferred embodiment, an apparatususing a static filter will additionally contain a diluter (coated withfused silica or a derivative thereof).

A sectional view of a preferred gas sample acquisition probe 100 andstatic filter housing 108 is shown in FIG. 3. The gas sample acquisitionprobe 100 is mounted to a stack wall 101 via a nipple 102. A sampleheating nipple 103 surrounds the portion of the gas sample acquisitionprobe 100 outside the stack 101. Between the gas sample acquisitionprobe 100 and the static filter housing 108 is a vaporization chamber104. A heater jacket 105 surrounds the static filter housing 108. Insidethe static filter housing 108 is a sintered stainless steel filter media106 having a fused silica coating 15. Also present on the static filterhousing 108 are a blowback 20, a calibration gas input 21, and a sampleoutput 22. The static filter housing 108 is kept in place by a specialtube fitting 107. All parts coming in contact with a gas samplepreferably are coated with a fused silica coating 15.

A plumbing diagram showing a sectional view of the gas sampleacquisition probe 100 and static filter housing 108 and three differentplumbing arrangements for the gas sample acquisition probe 100 andstatic filter housing 108 is shown in FIG. 7. The flow diagram indicatesthat flue gas flows up over the gas sample acquisition probe 100, whichcollects a gas sample. This gas sample enters the static filter housing108 having a calibration gas input 21 and a sample gas output 22. Themass flow controller 250 is used to accurately measure a calibration gasvolume injected into the sample flow.

Optimal Operating Conditions

It is desirable, but not required, to keep the inertial filter andassociated sampling components at around 200° C. to ensure optimumaccuracy in the measurement of total gaseous mercury concentration. Atthis temperature, the particulate material in the flue gas sampleappears to have minimal effect on the integrity of the gas sample.Adsorption of oxidized mercury appears to be minimized and oxidation ofelemental mercury to oxidized mercury is also minimized by the materialsand compounds in the flue gas at this temperature.

Preparation of Preferred Apparatus

Three sizes of the porous filter element are preferable: approximately½″, ⅜″, and ¼″ inner diameter (ID) of a sintered metal filter media orceramic filter media.

The entire flow path throughout the preferred apparatus is relativelysmooth, with no gaps in the tubing of the assembly where particulatematerial can collect. Accordingly, the preferred apparatus provides aconsistently laminar flow (smooth, undisturbed, quiet flow) of thesample past all of the inner tubing in contact with the flue gas sample.The particles are encouraged to move rapidly, in a vector parallel tothe tubing wall surfaces, with minimum impingement, especially in thearea of the porous filter element. In preferred embodiments, thereshould be no cracks, welds, or other disturbances in the continuity ofthe tubing in the flow path. Similarly, the sample gas temperature ismaintained at or above the flue gas temperature being sampled to preventmoisture condensation, and importantly, Hg⁰ and Hg⁺² deposition on thetubing walls. A temperature of approximately 200° C. or greater isparticularly preferred in this regard.

The preferred inertial filter apparatus has 5 temperature-controlledzones:

-   -   i. entire enclosure heaters;    -   ii. heated stinger extension;    -   iii. inertial filter body heater;    -   iv. eductor air preheater; and    -   v. diluter assembly, when fitted.

The heated filter probe extension, where the stinger, or gas sampleacquisition probe 100 (probe tubing extending into the stack) isattached, uses a tube union. The ID of the stinger matches the ID of theheated inlet tubing. Stingers are usually approximately 36″ toapproximately 80″ in length, coated with a fused silica 15 to preventdeposition of Hg⁰ and Hg⁺². This heated extension surrounds the sampleinlet tubing to heat the incoming gas to approximately 200° C. Theheated extension is approximately 6″ long or longer, and preferablyheated with one cartridge type heater, watt capacity sized to fit theapplication. For some applications where there is a stack liner, theheated extension runs the entire length between the mounting flange 156and the stack inside wall surface 101. The cartridge heater iscontrolled by a PID, thermocouple, or RTD controller, according to userpreference. For longer heated extensions, preheated air is circulatedinside the extension to maintain the temperature at approximately 200°C.

The incoming sample tubing 157 ID matches the ID of the porous filterelement 14. This tubing, approximately 15″ long, extends through theheated extension into the apparatus enclosure 155, ending in a tubeunion 158 attached to the porous interior of the inertial filter 10.Welded into this inlet tubing is an about ¼″ spike calibration gas inlettube, welded to the sample inlet tubing as close to the mounting flangearea as possible. There is an about 6 inch run of inlet sample tubingafter this weld tee, to allow for sufficient calibration gas mixing,prior to the stack gas/calibration gas mixture entering the porousfilter element 14 section of the inertial filter 10. Preferably, allmetal surfaces, including the stinger 100, tube unions (e.g., 158 and160), inlet tubing (157) run with calibration tee, porous filter element14, and inertial filter 10, are coated with fused silica 15 to preventabsorption or chemical changes in Hg⁰ to Hg⁺² ratios.

The porous filter element 14 length is nominally set from about 12 toabout 24 inches of filter length. This length can vary depending upontake-off sample flow rates desired by the analysis system. The sampletake-off axial velocity should not exceed about 0.006 fps. The fast loopflow design specifications for each size filter is:

-   -   i. ½″: 0 to 300 liters/minute;    -   ii. ⅜″: 0 to 200 liters/minute; and    -   iii. ¼″: 0 to 100 liters/minute.

The fast loop and bypass flow rates are calculated to produce 70-100 fpsgas velocity through the porous filter element 14. Particularlypreferred is an apparatus that maintains a 10,000:1 fast loop, bypass tosample take-off velocity.

A special inlet tubing fitting 43 centers the porous filter element 14into the inlet and outlet sample tubing to provide a uniform, smooth IDtube without any breaks or cracks, both coated with fused silica 15,prior to welding up the inertial filter 10. An outer tube 11 surroundsthe porous filter element 14, which has an outer diameter (OD) sized toprovide an about 0.0625″ annular space between outer tube ID and porousfilter element OD. All surfaces are coated with fused silica 15 prior towelding up the apparatus, which coating is preferably repeated afterwelding.

There are approximately five about ¼″ tubing taps on the inertial filterapparatus 10.

-   -   1. The first tap is the spike calibration gas tap 171 on the        inlet sample tubing 157 close to the mounting flange 156.    -   2. There are four about ¼″ tubing taps 41 on the side of the        annular tubing 11 surrounding the porous filter element 14.        These taps are for:        -   1. a blow back air inlet 20;        -   2. a calibration gas input 21;    -   3. a sample output 22; and    -   4. a filter surface temperature thermocouple or RTD 23.

All tubing downstream of the porous filter element 14 maintains thesmooth inner bore without breaks, cracks, or ID changes. These surfacesdo not have to be coated with fused silica since the sample has alreadybeen taken.

There is an about 180° loop in the tubing to return the fast loopbypassed sample to the front of the apparatus enclosure 155. The nextcomponent in the piping is a venturi 161, specifically designed for theflow ranges given above for a gas density of stack gas of approximately29 molecular weight, with upstream and downstream pressure taps toprovide a pressure drop output. Using NIST traceable calibration curve,the user can determine the total flow for the fast loop, bypassed samplegas flow. This factor is required to measure the dilution occurring whencalibration gas is injected into the inlet sample tubing about ¼″calibration inlet 171, for the “spike” calibration required by the EPAto assess the transport performance of Hg⁰ to Hg⁺² through the porousfilter element 14. Accordingly, this will allow for the measurement ofthe Hg⁰ to Hg⁺² mercury species only, and not the particulate boundmercury.

The venturi's 161 inlet and outlet connection are welded tubing, withthe inlet and outlet of the venturi 161 machined to accept a tubing buttfit for smooth bore flow, to prevent particulate material fromaccumulating.

Downstream of the venturi 161 is an air powered eductor 162 with thesame tubing ID and induced flow range, with about 100 psig supply airpressure, to produce the fast loop, bypass flue gas sample flow rateslisted above.

The motive power air passes through an air pre-heater 167, electricallypowered and controlled, with sufficient wattage to produce exit air at atemperature greater than about 150° C. to the eductor 162.

The purpose of the eductor air preheater 167 is to prevent watercondensation in the body of the eductor 162, which would result in stackgas particles agglomerating in the eductor 162, reducing flow andefficiency of the eductor 162.

All four (4) temperature-control zones installed in the Mercury InertialFilter Assembly have bimetallic overtemp switches for circuitprotection.

Blowback Assembly

In addition to high velocities through the porous filter element 14 ofthe inertial filter 10, a high-pressure air backpurge, or blowbackassembly, is installed.

All components of the blowback assembly are mounted within the heatedzone, except for the control solenoid (solenoids). These valves aremounted in an externally mounted enclosure 168 at ambient temperature.Included is a stainless steel pressure vessel, preferably about 1-liter,for accumulating back purge to a volume of about 10 liters at a pressureof about 90 psig.

The stored air, heated by the stainless tank mounted in the 200° C.enclosure zone, flows from storage through the two way blowback solenoidto the blowback tubing connection on the inertial filter 10. Thisblowback backpurges the filter.

Some applications may require backpurging the high speed, bypass samplepiping, and the gas sample acquisition probe 100. In this regard, a3-way solenoid preferably is mounted in the external enclosure 168 tocontrol the direction of the hot backpurge air either to the porousfilter blowback fitting, or to the calibration gas spike tubing fitting.

Sample Pump

A diaphragm heated head sample pump 164 is mounted integral within theapparatus enclosure 155. All surfaces in contact with the sample areeither Teflon® or fused silica coated stainless steel or Hastelloy. Anextended head from the sample pump 164 extends into the heated zone tomaintain an operating head temperature of at least about 200° C.

The sample pump 164 is connected directly to the sample out tubing 22mounted on the inertial filter 10. Occasionally, a particulate filter,sub micron, glass, is fitted in the tubing 159 between the sample outtubing 22 and the sample pump 164 inlet. In a speciating Hg inertialfilter apparatus, a Hg⁺² reducer is fitted in between the inertialfilter sample outlet and the sample pump inlet.

When a Diluter Assembly 201 is fitted to the inertial filter apparatus10, the sample pump 164 is not fitted. The Hg⁺² reducer is fitted afterthe Diluter Assembly 201 to decrease the reduction load on the reducerassembly.

Mercury Inertial Filter Assembly Outlet Connection

The inertial filter 10 is fitted with a heated sample line bootassembly. This sample line boot assembly can be about 1½″ to 2½″ indiameter, and has a heat shrink section to fuse to the heated sampleline surface, sealing the apparatus enclosure 155 from ambient air. Allfield connections to the inertial filter 10 enter the heated zonethrough this boot. The heated sample line contains: an about ¼″ PFATeflon® sample line; an about ¼″ PFA Teflon® air supply line; an about¼″ PFA Teflon® calibration gas line; messenger wires to power theassembly; and digital messenger wires for monitoring and control.

The PFA Teflon® sample line is connected to the sample out tubingconnector mounted on the sample pump head.

Diluter Assembly for the Hg Inertial Filter Assembly

Some process and flue gas applications require a dilution of theexisting Hg⁰ to Hg⁺² species. Dilution of the sample gas occurs in thisdiluter assembly 201, in ratios ranging from about 13:1 to about 250:1.

All metallic surfaces of the diluter assembly 201 are coated with fusedsilica 15 to prevent mercury speciation changes or adsorption.

The diluter assembly 201 is connected directly to the inertial filteroutlet tubing using a fused silica 15 coated tube union. The diluterassembly 201 consists of a glass precision orifice, mounted in a holderwith an “O” ring and tube fitting. This sample precision orifice ismounted directly onto the body of the dilution eductor.

The diluted sample then exits the inertial filter 10 via a fused silica15 coated bulkhead fitting. Tubing from the outlet of the dilutioneductor to the bulkhead fitting is PFA Teflon®.

Calibrators

Calibrators for generating Hg⁰ to Hg⁺² concentrations directly into theinertial filter 10 are fitted directly onto the exterior surface of theheated inertial filter 10.

Oxygen Sensor

Under certain circumstances, either by application, EPA Federalrequirement, or local APCD demand, an oxygen sensor (Diluent Analyzer)may be installed in the inertial filter 10.

This Diluent Analyzer is mounted integrally within the apparatusenclosure 155 or directly attached to the exterior surface of theapparatus enclosure 155.

Depending upon application, the measurement of this oxygen sensor willbe on either a wet basis or a dry basis, using a thermo-electric (TE)chiller.

Sample Transport and Conditioning System.

Dry Basis Sample Transport and Conditioning System

A heated sample line, operating at a temperature of about 200° C.,connects the inertial filter 10 with the remaining sample conditioningsystem. This heated sample transport line terminates at a thermoelectric(TE) sample gas chiller to remove water. The TE chiller is fitted withremovable impingers, and all surfaces are coated with fused silica 15,and sealed with a Viton “0” ring. The impingers are designed to rapidlyseparate water from sample gas to minimize mercury contact with thecondensed water.

Water is removed via a peristaltic drain pump with tubing and RPM sizedto fit the application. The sample gas exits the TE chiller and passesto the downstream mercury analysis system. No flowmeters, filters, orother components are fitted into the flow stream, in order to minimizecomponents and surface area, which results in Hg⁰ loss.

Wet Basis Sample Transport System

When a diluter is fitted into the inertial filter 10, no other sampleconditioning components are required. A frost-free, ¼″ PFA Teflon®sample line is connected between the inertial filter 10 and thedownstream mercury analysis system.

A preferred heated gas sample acquisition and static filtrationapparatus is designed for mounting on a stack or duct. Its primaryfunction is to collect a flue gas sample, provide a heated environmentto maintain sample gas temperatures above dewpoint, and removeparticulate material from the gas sample. This heated gas sampleacquisition and static filtration apparatus contains a standard 10 or 50micron sintered stainless steel filter media, a circuit board regulatedheater jacket, an integral calibration gas port on both sides of thefilter media, a NEMA 4 enclosure, and a circuit board controlledblowback system to clean the filter media.

The general specifications for this heated gas sample acquisition andstatic filtration apparatus are as follows:

Probe 18″ Stinger probe, 0.5″ diameter × 0.065″ wall, 316 L tubingCalibration Integral calibration on both sides of filter element HeaterJacket Circuit board regulated Connections 1¼″ male pipe nipple mount;½″ male pipe thread adapter Connectors ¼″ cal gas, ¼″ sample lineThermocouple Type K Blowback Single direct; 2-way solenoidblowback/calibration valve Blowback Tank 16 gallon stainless steel, 4″ ×8″, leak checked, pressure tested Heat-shrink 7″ length, 2.75″ minimumexpanded ID nose Boot O-rings Viton ® Gaskets Graphoil or SiliconeDimensions 14″ × 12″ × 8″ HWD (w/out Stinger probe) Weight 34 lbs.

Similarly, the operating specifications for this heated gas sampleacquisition and static filtration apparatus are as follows:

Sample Flow Rate 6-10 LPM Calibration Gas Requirement 20 psig, 6-10 LPMProbe Operating Temperature 375° F. (190° C.) Input Voltage 90-260 VAC,50/60 Hz Blowback Duration 5 sec standard (30 sec maximum) BlowbackFrequency Every 24 hours standard (range 10 minutes to 99 hours)Blowback Valve 12 or 24 VDC, 115 or 230 VAC, 100 psig max pressureBlowback Flowrate 14 scfh Instrument Air for Blowback Min 50 psig, Max90 psig

Likewise, the material specifications for this heated gas sampleacquisition and static filtration apparatus are as follows:

Enclosure Material NEMA 4 Heater Type Silicone rubber blanket with snapclosures Oven Insulation Material ⅛″ thick silicone, medium densityFilter Chamber Material 316 stainless steel Filter Element Types 2micron ceramic 5, 10, 20 micron sintered SS 2 micron SS screen mesh

The probe is preferably coated with a fused silica or derivative thereofprotective coating to prevent corrosion, gas absorption, or gas reactionwith the stainless steel construction of the probe.

Gas Analyzer

Once the gas sample has been collected and conditioned by the preferredgas sample acquisition and filtration apparatus, the sample will pass toa gas analyzer, where the total gaseous mercury concentration, includingconcentration of Hg⁰ and Hg⁺² individually, is measured. Suitable gasanalyzers are well known in the art and include, without limitation, UVatomic absorption or atomic fluorescence detectors.

Preferred Processes

A preferred aspect further relates to a process for accurately measuringtotal gaseous mercury concentration in flue gas comprising the steps of:

-   -   (1) removing a gas sample from flue gas containing gaseous        mercury using a gas sample acquisition probe;    -   (2) passing said gas sample into an inertial filter comprising a        porous filter element to remove particulate material present in        said gas sample, wherein all metal surfaces of said inertial        filter are coated with a protective coating; and    -   (3) measuring the amount of gaseous mercury present in said gas        sample.

In a preferred embodiment, all metal surfaces of the gas sampleacquisition probe coming into contact with the flue gas are coated witha protective coating. In a preferred embodiment, the protective coatingis comprised of fused silica or a fused silica derivative. Theprotective coating can have a thickness of up to about several thousandAngstroms. Further, the protective coating reduces chemical reactivityto the gaseous mercury of all of the metal surfaces coming into contactwith the gas sample without impeding gas flow through the inertialfilter. The process chemical reactions reduced on said metal surfacesaccording to the preferred processes comprise oxidation of elementalmercury to a chemically combined form of mercury and loss of oxidizedmercury through adsorption on said metal surfaces. Accordingly, thepreferred processes do not chemically alter gaseous mercury present inthe gas sample passing through the inertial filter.

In another preferred embodiment, these processes permit the measurementof gaseous mercury selected from the group consisting of Hg⁺², Hg⁰, anda combination thereof. Accordingly, these processes are capable ofdetermining how much of the total gaseous mercury concentration ispresent in the flue gas as Hg⁺² and how much of the total gaseousmercury concentration is present in the flue gas as Hg⁰.

Preferred processes insert a gas sample acquisition probe (stinger) intothe flue gas to take the gas sample. This gas sample is then maintainedat a temperature greater than or equal to the temperature of the fluegas. The gas sample can be taken from the flue gas before the flue gaspasses through an electrostatic precipitator (ESP); after the flue gaspasses through an ESP; before the flue gas passes through a wetscrubber; after the flue gas passes through a wet scrubber; before theflue gas passes through a flue gas desulfurization device (FGD); afterthe flue gas passes through an FGD; or from final flue gas passingthrough a smoke stack. Accordingly, preferred processes accuratelymeasure total mercury concentrations, both Hg⁰ and Hg⁺², in a gas sampletaken from any of these sampling locations typically found in coal firedpower plant processes.

EXAMPLES

Numerous tests have verified the unexpectedly advantageous properties ofthe preferred apparatus, most notably those conducted by Sjostrom et al.relating to an initial version of a preferred apparatus. In some ofthese tests, coated parts have been substituted for uncoated parts inmercury sampling systems as a basis for comparison. In all cases, thetotal gaseous mercury recovered improved and the integrity of the ratioof elemental to oxidized mercury was better preserved when exposed tothe coated parts.

Two systems were used to introduce elemental and oxidized mercury intothe probe extension upstream of the gas sample acquisition andfiltration apparatus during testing. The elemental spiking system was aPSA 10.534 Mercury Calibration System fabricated by PS Analytical. ThePSA 10.534 provides a wide range of mercury concentrations by alteringthe temperature of a mercury reservoir and varying the gas flowratethrough the reservoir. The mercury reservoir is constructed byimpregnating elemental mercury on an inert substrate. The gas passingover the reservoir becomes saturated with mercury at the reservoirtemperature.

The oxidized mercury source was a Hot-Vapor Calibration (HOVACAL) systemmanufactured by IAS GmbH. For this system, a solution of mercuricchloride is injected onto a heated head to evaporate the solution.Nitrogen carries the gaseous mercuric chloride into the bulk gas. Theliquid calibration solution is delivered to the head with a peristalticpump. The flow rate of the pump is confirmed using a loss-of-weightbalance. The peristaltic pump feed rate, the nitrogen flow rate, and thehead temperature are controlled with the touch-screen interface shown onthe front of the HOVACAL unit.

The goal of the evaluation program was to test three inertial probes inorder to determine the effectiveness of the inertial filter to transportboth elemental and oxidized mercury in a relatively clean gas stream.The inertial probe apparatuses tested were:

A) Large Inertial Probe apparatus with fused silica coating on allsample contact surfaces.

B) Large Inertial Probe apparatus with no fused silica coating.

C) Small Inertial Probe apparatus with no fused silica coating.

A further goal of this evaluation program was to quantitatively assessthe ability to inertially separate particles with a high affinity formercury with minimal sampling artifacts. The general plan was to spikethe probes with elemental mercury and measure quantitatively how much ofthe mercury passes through the filter.

Probe Preparation

Flow Measurement

The oxidized and elemental spiking systems were configured to introducea small volume, concentrated stream of vapor-phase mercury into the bulkprobe flow upstream of the inertial filter. The flow through the filtermust be monitored to determine the predicted mercury concentration inthe bulk flow downstream of spiking.

Each apparatus was retrofitted with a venturi flow meter prior toinstallation to monitor the flow through the apparatus. The venturi flowmeters were added to the end of the exhaust return pipe on the apparatusto eliminate the need to modify the overall apparatus design oroperation. Compressed air is introduced into the vacuum eductorsupstream of the exhaust venturis. Therefore, the compressed air flow wasalso monitored to determine the flow through the inertial filter portionof the gas sample acquisition and filtration apparatus. Two of theapparatuses also included integral venturi flow meters.

Calibration Ports

The Apogee and Thermo apparatuses were retrofitted with calibration tapsimmediately upstream of the inertial filter to allow spiking withelemental and oxidized mercury. The length of the calibration line wasminimized to prevent losses of HgCl₂ in calibration line upstream of theapparatus.

Probe Extensions (Stingers)

A 6-foot fused silica coated stinger was installed on the inlet of eachextraction probe. For the two higher flow apparatuses, ¾″ pipe was used.One-half inch tube was used for the stinger on the lower-flow apparatus.

Activated Carbon Injection Line

A ¼-inch stainless steel carrier line was installed along the stinger.One end of this line extended into the tip of the stinger. The oppositeend of this line terminated at the flange to allow doping with activatedcarbon.

Test Protocol

The exhaust venturi flow meters and integral venturi flow meters werecalibrated using a laminar flow element prior to testing. Thesecalibrations were used to determine the expected vapor-phase spikeconcentrations during elemental and oxidized spiking periods. Theapparatuses were installed and operating at the manufacturersrecommended operating conditions (flow and temperature) at least 24hours before initiating testing.

At the beginning of each test day, the instruments were calibrated withthe on-board permeation device. Following calibration, baseline mercurymeasurements were made to establish the vapor-phase mercuryconcentration and speciation in the duct. A PRB coal was tested. Themercury concentration was fairly stable and the mercury was primarily inthe elemental form (typically >80%).

After establishing the baseline conditions, the output from the PSA10.534 was connected to the calibration port on the extraction probe.Based upon the saturator temperature and flowrate through the PSA10.534, the mercury introduced to the probe (ng/min) could becalculated. The concentration of mercury expected was determined byadding the mercury mass injection rate divided by the probe flow to thebaseline duct concentration. Each apparatus was evaluated at 300 and400° F.

Following performance evaluations with elemental mercury, theapparatuses were spiked with oxidized mercury from the HOVACAL system.The concentration of oxidized mercury was calculated using the injectionrate of the HgCl₂ solution and the flow rate through the apparatus.Tests were conducted at 300 and 400° F. to establish the temperaturestability of the apparatuses for oxidized mercury measurements.

The final test was designed to determine the inertial separationeffectiveness. Because the three apparatuses were installed at theoutlet of the electrostatic precipitator, the fly ash loading to theapparatuses was quite low. This provided a good opportunity to evaluatethe apparatus for elemental and oxidized mercury measurements withoutconcern for in-duct reactions with the fly ash. However, this testlocation and particular fly ash (relatively low mercury removaleffectiveness) did not challenge the apparatuses for their ability toadequately separate particulate matter while minimizing the samplingartifacts.

For the final test, NORIT FGD activated carbon was introduced by batchinjection into the tip of the probe extension. The injection rate wasequivalent to nominally 45 lb/MMacf, which is roughly twice the maximuminjection rate economically feasible for an ESP application. Theinjection was maintained for 30 seconds and repeated at two differentapparatus operating temperatures (300 and 400° F.). Total and elementalmercury measurements were made during and following injection. Theresidence time in the probe extensions was less than 0.15 seconds formost of the test conditions.

Results

The baseline vapor-phase mercury concentration in the flue gas duringtesting ranged from 9 to 12 μg/Nm³. The flue gas flowrate through thelarger apparatuses ranged from 10 to 11.5 acfm. The flowrate through thesmaller apparatus ranged from 2.25 to 3.0 acfm. The large apparatuseswere operated at their maximum design velocity and the small apparatuswas operated slightly above the design velocity of the system.

Elemental Mercury Spiking

All three apparatuses performed well when elemental mercury wasintroduced upstream of the inertial filter. The recovery of totalmercury was quite good and well within the measurement uncertainty ofthe apparatus flow. Both uncoated stainless steel apparatusesdemonstrated some oxidation. The larger apparatus demonstrated somewhatmore oxidation (11% at 300° F. and 19% at 400° F.). The smaller uncoatedapparatus demonstrated slight (3 to 5%) oxidation at 300 to 400° F. Thefused silica coated apparatus did not demonstrate any measurableoxidation at either temperature.

Oxidized Mercury Spiking

High concentrations of oxidized mercury can be difficult to transport.During spiking with oxidized mercury, care was taken to assure spiketransport lines were as short as possible and to maintain the transportlines at the temperature of the apparatus. The spiking port on two ofthe apparatuses (the small apparatus and one of the larger apparatuses)was retrofitted so that the HOVACAL head could be connected directly tothe port. The calibration port for the other large apparatus was locatedinside the heated enclosure requiring nominally 18-inches of PFA Teflon®and 6-inches of coated stainless steel transport line prior to enteringthe bulk probe flow.

The recovery of oxidized mercury was very good (>90% of the expectedconcentration) on both apparatus that could be close coupled to theHOVACAL head. For the larger apparatus, the recovery was slightly lowerat 300° F. than 400° F. There was no measurable difference inperformance between 300 and 400° F. for the smaller apparatus. Thesmaller apparatus also resulted in a higher fraction of the samplemeasured as oxidized (>95% of expected). The oxidized mercury measuredwith the larger apparatus was 82 to 85% of the expected concentration.

The oxidized mercury spike test for the apparatus with the calibrationport extension was not as successful as the other two apparatuses.Nominally 50% of the spiked concentration was measured when using thisapparatus, and only 30 to 40% of the expected concentration was measuredas oxidized mercury. Due to the difficulties transporting oxidizedmercury, it is suspected that the poor performance was due todifficulties transporting the spike sample to the calibration port onthe apparatus and not necessarily due to any other aspect of theapparatus design. Modifications to allow direct coupling of the HOVACALhead to this apparatus are required to allow an appropriate evaluationof oxidized mercury spiking.

Doping with Activated Carbon

Each of the apparatuses was challenged with activated carbon followingcharacterization with elemental and oxidized mercury to qualitativelyassess the ability of the apparatuses to inertially separate particulatematter with an affinity for mercury.

Both large apparatuses demonstrated a rapid drop in mercuryconcentration following injection. The concentration immediatelyfollowing injection was 15 to 60% lower than the initial concentration.The total mercury concentration on one of the large (uncoated)apparatuses returned immediately to the baseline concentration followinginjection. However, some activated carbon remained on the filter asevidenced by the increased oxidation across the apparatus (typically 20%additional oxidation following injection).

The total mercury concentration measured with the coated apparatus didnot immediately return to baseline concentrations but remainedsuppressed to up to an hour following doping with powder activatedcarbon (PAC). This PAC is iodized to enhance its affinity for Hgspecies. The PAC is intentionally injected to adsorb Hg species onto itssurface, and be removed in a downstream COHPAC baghouse, and finallydiscarded with the flash. The fraction of oxidized mercury alsoincreased across this probe following each injection episode. Thepreferred apparatus should not hang up carbon as the operator will usethe inertial filter to control the carbon injection rate.

No drop in mercury concentration or increase in oxidation was notedacross the small apparatus.

It is important to note, however, that this activated carbon test doesnot accurately represent typical plant operating conditions under whichthe preferred apparatus would be expected to perform. Most power plantoperators do not want to emit any excess carbon as this includesunburned fuel. In fact, many utilities have monitors to detect andcontrol carbon levels. Accordingly, the preferred apparatus, when usedin such industrial applications, would avoid significant problemsrelated to mercury being trapped on excess carbon.

Additionally, this test was conducted on an embodiment of the presentapparatus that did not contain the unions 43. A preferred apparatuscontaining the unions 43 would not exhibit this Hg attenuation.Accordingly, the above-described area that collected carbon would not bepresent in the preferred apparatus.

The foregoing is a detailed description of preferred embodiments. Itwill be apparent that the same may be modified or varied in many ways.Such modifications and variations are not to be regarded as a departurefrom the spirit and scope of the inventive subject matter, and all suchmodifications and variations are intended to be included within thescope of the following claims.

1. An apparatus for measuring mercury concentration in a flue gas, theapparatus comprising a gas sample acquisition probe, an inertial filter,and an analyzer, wherein: the gas sample acquisition probe is configuredto initiate transport of a gas sample from the flue gas to the inertialfilter; the inertial filter comprises a porous filter element thatremoves particulate material from the gas sample and outputs a filteredgas sample; the analyzer measures the amount of gaseous mercury presentin the filtered gas sample; the inertial filter comprises metal surfacesthat are coated with a protective coating that reduces chemicalreactivity of the metal surfaces to gaseous mercury; the protectivecoating comprises fused silica or a derivative thereof and the coatedmetal surfaces are further coated with a secondary chemical deactivationlayer configured to cover silanol groups on the surface of theprotective coating; and the coated filter surfaces are positioned tocontact a gas sample flowing through the filter.
 2. An apparatus formeasuring mercury concentration in a flue gas, the apparatus comprisinga gas sample acquisition probe, an inertial filter, and an analyzer,wherein: the gas sample acquisition probe is configured to initiatetransport of a gas sample from the flue gas to the inertial filter; theinertial filter comprises a porous filter element that removesparticulate material from the gas sample and outputs a filtered gassample; the analyzer measures the amount of gaseous mercury present inthe filtered gas sample; the inertial filter comprises metal surfacesthat are coated with a protective coating that reduces chemicalreactivity of the metal surfaces to gaseous mercury; the protectivecoating comprises a polymer; and the coated filter surfaces arepositioned to contact a gas sample flowing through the filter.
 3. Anapparatus as claimed in claim 2 wherein the polymeric protective coatingcomprises a plurality of polymeric layers bonded to the metal surfaces.4. An apparatus for measuring mercury concentration in a flue gas, theapparatus comprising a gas sample acquisition probe, an inertial filter,and an analyzer, wherein: the gas sample acquisition probe is configuredto initiate transport of a gas sample from the flue gas to the inertialfilter; the inertial filter comprises a porous filter element thatremoves particulate material from the gas sample and outputs a filteredgas sample; the analyzer measures the amount of gaseous mercury presentin the filtered gas sample; the inertial filter comprises metal surfacesthat are coated with a protective coating that reduces chemicalreactivity of the metal surfaces to gaseous mercury; the protectivecoating has a thickness of up to approximately several thousandAngstroms; and the coated filter surfaces are positioned to contact agas sample flowing through the filter.
 5. An apparatus for measuringmercury concentration in a flue gas, the apparatus comprising a gassample acquisition probe, an inertial filter, and an analyzer, wherein:the gas sample acquisition probe is configured to initiate transport ofa gas sample from the flue gas to the inertial filter; the inertialfilter comprises a porous filter element that removes particulatematerial from the gas sample and outputs a filtered gas sample; theanalyzer measures the amount of gaseous mercury present in the filteredgas sample; the inertial filter comprises metal surfaces that are coatedwith a protective coating that reduces chemical reactivity of the metalsurfaces to gaseous mercury; the thickness of the protective coating islimited to a few molecules; and the coated filter surfaces arepositioned to contact a gas sample flowing through the filter.
 6. Anapparatus for measuring mercury concentration in a flue gas, theapparatus comprising a gas sample acquisition probe, an inertial filter,and an analyzer, wherein: the gas sample acquisition probe is configuredto initiate transport of a gas sample from the flue gas to the inertialfilter; the inertial filter comprises a porous filter element thatremoves particulate material from the gas sample and outputs a filteredgas sample; the analyzer measures the amount of gaseous mercury presentin the filtered gas sample; the inertial filter comprises metal surfacesthat are coated with a protective coating that reduces chemicalreactivity of the metal surfaces to gaseous mercury; the thickness ofthe protective coating is such that the porous filter element definespore size from about 0.1 microns to about 50 microns absent theprotective coating and a pore size from 0.1 microns to about 0.5 micronswith the protective coating; and the coated filter surfaces arepositioned to contact a gas sample flowing through the filter.
 7. Anapparatus for measuring mercury concentration in a flue gas, theapparatus comprising a gas sample acquisition probe, an inertial filter,and an analyzer, wherein: the gas sample acquisition probe is configuredto initiate transport of a gas sample from the flue gas to the inertialfilter; the inertial filter comprises a porous filter element thatremoves particulate material from the gas sample and outputs a filteredgas sample; the analyzer measures the amount of gaseous mercury presentin the filtered gas sample; the inertial filter comprises metal surfacesthat are coated with a protective coating that reduces chemicalreactivity of the metal surfaces to gaseous mercury; the chemicalreactivity of the metal surfaces that is reduced comprises oxidation ofelemental mercury to a chemically combined form of mercury and loss ofoxidized mercury through adsorption on the metal surfaces; and thecoated filter surfaces are positioned to contact a gas sample flowingthrough the filter.
 8. An apparatus for measuring mercury concentrationin a flue gas, the apparatus comprising a gas sample acquisition probe,an inertial filter, and an analyzer, wherein: the gas sample acquisitionprobe is configured to initiate transport of a gas sample from the fluegas to the inertial filter; the inertial filter comprises a porousfilter element that removes particulate material from the gas sample andoutputs a filtered gas sample; the analyzer measures the amount ofgaseous mercury present in the filtered gas sample; the inertial filtercomprises metal surfaces that are coated with a protective coating thatreduces chemical reactivity of the metal surfaces to gaseous mercury;the protective coating prevents chemical alteration of the gaseousmercury present in the sample gas passing through the inertial filter;and the coated filter surfaces are positioned to contact a gas sampleflowing through the filter.
 9. An apparatus for measuring mercuryconcentration in a flue gas, the apparatus comprising a gas sampleacquisition probe, an inertial filter, and an analyzer, wherein: the gassample acquisition probe is configured to initiate transport of a gassample from the flue gas to the inertial filter; the inertial filtercomprises a porous filter element that removes particulate material fromthe gas sample and outputs a filtered gas sample; the analyzer measuresthe amount of gaseous mercury present in the filtered gas sample; theinertial filter comprises metal surfaces that are coated with aprotective coating that reduces chemical reactivity of the metalsurfaces to gaseous mercury; the gaseous mercury measured in theparticulate free gas sample is selected from the group consisting ofHg⁺², Hg⁰, and a combination thereof; and the coated filter surfaces arepositioned to contact a gas sample flowing through the filter.
 10. Anapparatus for measuring mercury concentration in a flue gas, theapparatus comprising a gas sample acquisition probe, an inertial filter,and an analyzer, wherein: the gas sample acquisition probe is configuredto initiate transport of a gas sample from the flue gas to the inertialfilter; the porous filter element comprises a ceramic filter media; theinertial filter comprises a porous filter element that removesparticulate material from the gas sample and outputs a filtered gassample; the analyzer measures the amount of gaseous mercury present inthe filtered gas sample; the inertial filter comprises metal surfacesthat are coated with a protective coating that reduces chemicalreactivity of the metal surfaces to gaseous mercury; and the coatedfilter surfaces are positioned to contact a gas sample flowing throughthe filter.
 11. An apparatus for measuring mercury concentration in aflue gas, the apparatus comprising a gas sample acquisition probe, aninertial filter, an analyzer, and a diluter assembly, wherein: the gassample acquisition probe is configured to initiate transport of a gassample from the flue gas to the inertial filter; the inertial filtercomprises a porous filter element that removes particulate material fromthe gas sample and outputs a filtered gas sample; the analyzer measuresthe amount of gaseous mercury present in the filtered gas sample; theinertial filter comprises metal surfaces that are coated with aprotective coating that reduces chemical reactivity of the metalsurfaces to gaseous mercury; the coated filter surfaces are positionedto contact a gas sample flowing through the filter; the diluter assemblyis coupled to the inertial filter and configured to dilute the samplegas; and the diluter assembly comprises metal surfaces that are coatedwith a protective coating that reduces chemical reactivity of the metalsurfaces to gaseous mercury.
 12. An apparatus for measuring mercuryconcentration in a flue gas, the apparatus comprising a gas sampleacquisition probe, an inertial filter, and an analyzer, wherein: the gassample acquisition probe is configured to initiate transport of a gassample from the flue gas to the inertial filter; the inertial filtercomprises a porous filter element that removes particulate material fromthe gas sample and outputs a filtered gas sample; the analyzer measuresthe amount of gaseous mercury present in the filtered gas sample; theinertial filter comprises metal surfaces that are coated with aprotective coating that reduces chemical reactivity of the metalsurfaces to gaseous mercury; the inertial filter comprises a pluralityof individual filter particles and the protective coating is provided soas to coat each of the individual filter particles; and the coatedfilter surfaces are positioned to contact a gas sample flowing throughthe filter.